Seismically induced shale diapirism: the Mine d’Or section, Vilaine estuary, Southern Brittany

Abstract

The Pénestin section (southern Brittany) presents large regular undulations, commonly interpreted as evidence of periglacial pingos. It is an upper Neogene palaeoestuary of the Vilaine River reactivated during the middle Quaternary (middle terrace). It is incised into a thick kaolinitic saprolite and deformed by saprolite diapirs. This paper presents the arguments leading to a mechanistic interpretation of the deformations at Pénestin. Neither recent transpressive tectonics nor diagnostic evidence of periglacial pingo have been found despite evidence for a late paleo-permafrost. The major deformational process is shale diapirism, initially triggered by co-seismic water supply, with further loading and lateral spreading on an already deformed and deeply weathered basement, which allowed the shale diapirism to develop. Deformations are favoured by the liquefaction of the saprolite and a seaward mass movement and recorded, rather distant, effects of an earthquake (c. 280 ka B.P.) resulting from the progressive subsidence of the southern Armorican margin. These deformations triggered by an earthquake are similar to those induced by classical shale diapirism. They are probably common in tectonically active continental environments with shallow water table.

Middle Pleistocene seismically induced clay diapirism in an intraplate zone, western Brittany, France

“…Further vertical motion of the Crozon Peninsula did not occur during the Plio-Quaternary, as proved by an Upper Miocene low-stand platform preserved in several places close to the present-day sea level, at Porz Kubu (Fig. 1B) Le Rhys, close to Douarnenez (Van Vliet-Lanoë et al, 2002), and at Pénestin (Vilaine River estuary; Van Vliet-Lanoë et al, 2009) (Fig. 2).…”

“…The stratigraphy for the Gurp section follows O’Brien and Jones (2003) and Sitzia et al (2015). Those of Pénestin are extracted from Van Vliet-Lanoë et al (1997) and Van Vliet-Lanoë et al (2009). ESR, electron spin resonance; IRSL, infrared-stimulated luminescence; MIS, Marine Oxygen Isotope Stage; NGF, French Ordnance Survey (5 m NGF corresponding to above sea level); SASZ, South Amorican Shear Zone.…”

“…Dating of the Quaternary units was previously performed by electron spin resonance (ESR) methods on the Trez Rouz, Pen Hat, and Penestin sections (M. Laurent, in Van Vliet-Lanoë et al, 1997, 2009). New infrared-stimulated luminescence (IRSL) ages on Marine Oxygen Isotope Stage (MIS) 6 deposits were performed on the Pen Hat section (Table 2).…”

“…These processes also allowed (1) an incipient swell or doming that stretched the overburden cover (Powley, 1999), (2) mud injected close to its apex within the confining layer, and (3) an upward stretching and thinning of the flanking units (Figs. 5 and 8A;Van Vliet-Lanoë et al, 2009). Youd et al (2004) showed that ground oscillations during an earthquake led to a continual rise of pore water pressure after ground shaking ceases, triggering the prolonged postshock deformation.…”

“…Clay diapirism is rather common in Europe, for instance, in the Oligocene Bovey-Tracey basin located in Devon in the United Kingdom along the Sticklepath Fault ( Fig. 2; Van Vliet-Lanoë et al, 2009), and in the lignite grabens in Germany and Poland (Eissmann, 1987). Site effects (Semblat et al, 2005) on unconsolidated sedimentary bodies certainly amplified the likelihood of diapirism in all the studied sites near Variscan structures.…”